Cannot remove files from my phone's micro sd card?

Okay, so here's my problem, hope somebody can help me out. First, the card I am using is a SanDisk micro SD HC 4 gig card; the device I am using it in is a Samsung Blackjack, SGH-i607 smart phone, but the operating system of the phone has been upgraded to Windows Mobile 6. My computer is a custom desktop, using Windows XP Home, with all the latest updates and service packs installed, and I am using Microsoft ActiveSync 4.5 to sync up my phone. For some reason I cannot delete certain files off of my sd card, either directly from the control of the phone or through exploring the phone's and card's files through my computer. I cannot see these files in any of the folders that I can't delete, but every time I try to delete the folders it says I don't have access to certain files and to confirm that they are read-only, but I can't even see any files. Any suggestions would be appreciated.